I think berserker too, or maybe a really ambitious crawler. Regardless, I think it's going to be an ex-zombie in 2 panels. The big question will be who gets the kill. I'm going with Sam because of the range of the shotgun over the shovel, Charlie seems a little stunned to act (and there's my double agent theory).

I'm going to put a guess in on Hunter due to the lack of Zerker grin another mentioned, though I did think Berzerker at first. But a Hunter springing out of ambush could fit, as well as explaining the lack of any sounds from within when he was listening.